This invention relates to an apparatus and method for creating exceptionally small holes primarily of the low micron or sub-micron diameter range, at very high rates of speed, typically on the order of 30,000 holes per second per laser used, in metallic or non-metallic foil or sheet, by means of pulsed laser energy, to provide an unusually high percentage of openness, typically on the order of 40%. For purposes of discussion and illustration herein, a pattern of 10 micron (0.00039 inch) diameter holes will be set forth, it being understood that the invention described is capable of accuracies required to create, at similar rates of speed, patterns of holes of much larger or much smaller size. Such microperforated foil finds ready and extensive application in a great many industrial and scientific circumstances, notably as filter media.
Since it is desirable that filter media offer a minimum impediment to fluid flow through the filter element, the hole pattern in the media should be spaced as closely as possible. For a pattern of 10 micron diameter holes, in order to achieve 40% openness, hole center-to-center distance must be 0.00058 inch, and holes in adjacent rows must be "nested" so that bridges between neighboring holes have a maximum width of 0.00019 inch and remain intact. It will be evident that hitherto conventional tooling and methods (such as punches and dies or electrical discharge) cannot create such hole patterns nor attain the requisite accuracy at high enough rates to be economically competitive with existing woven media. For example, the number of holes that must be placed per square foot in the 10 micron pattern cited is approximately 482 million. In a pattern of four micron diameter holes, a 40% open condition demands more than three billion holes per square foot. Therefore, any process of manufacture, to be economically feasible, must be capable of very high rates of hole production as well as a high degree of repetitive accuracy in hole placement. In the present invention as herein described, 30,000 holes per second per laser is an expected minimum production rate for 10 micron diameter holes; in the case of four micron diameter holes, it is 45,000 holes per second per laser. At such rates, these foil filter media can be manufactured at a fraction of existing costs, and with a significantly greater percentage of openness (about 40% vs. 33%).
The wavelength of the laser and the mode in which it oscillates control the minimum diameter to which the laser beam can be focused. The smallest spot diameter will be realized when the laser operates in the fundamental, i.e. TEM.sub.00, mode, but in higher order modes available laser power is greater. It has been found that oscillation in the TEM.sub.01 mode, in which significantly greater power is available, will still permit focusing laser energy of 1.06 microns wavelength to a 10 micron diameter, providing that a "beam expander" is included with the focusing optics. These facts may permit a rate of hole production greater than 30,000 per second, which figure is based on an 18 watt laser operating in fundamental mode.
There is wide latitude in the type of laser than can be used. Choice will depend on the size of holes desired, minimum energy needed per pulse, mode of oscillation used, repetition rate required. In the example of 10 micron diameter holes, a convenient wavelength of 1.06 microns can be obtained from a Nd:YAG or equivalent laser type. For holes much smaller than 10 microns, a laser of shorter wavelength would be chosen.
In order to create holes at the described rate of 30,000 per second, not only must the laser beam be switched on and off at that rate, but the relative position of the laser beam and the material being perforated must be changed to accomodate that rate while simultaneously maintaining the necessary locational accuracy required by the hole pattern. The use of a laser to create holes by means of focused laser radiation is common. However, no process to date is able to attain the accuracy of hole placement together with the continuous, very high rate of hole production required to equal or better the woven filter media in performance and cost.